Increasing a flow rate of oil into a compressor of a refrigeration assembly

ABSTRACT

A method includes receiving, by a processing device and from a variable frequency drive coupled to one or more compressors, operation information of the one or more compressors. The method also includes comparing the operation information of the one or more compressors to an operation threshold and determining that the operation information satisfies the operation threshold. The method also includes changing, based on the determination that the operation information of the one or more compressors satisfies the operation threshold, an operation parameter of a component of the refrigeration system. Changing the operation parameter increases at least one of: (i) a velocity of a working fluid in a piping assembly fluidly coupled to the one or more compressors, or (ii) a flow rate of an oil in the piping assembly flowing into the one or more compressors.

FIELD OF THE DISCLOSURE

This disclosure relates to refrigeration systems, and particularly tooil management in refrigeration systems.

BACKGROUND OF THE DISCLOSURE

Refrigeration systems are used to cool spaces such as refrigerators,freezers, coolers, and display cases. Refrigeration systems rely onrefrigeration cycles of a refrigerant that alternately absorbs andrejects heat as the refrigerant is circulated through the system.Refrigeration systems include one or more compressors that compress theworking fluid to increase the pressure of the fluid as part of therefrigeration cycle. Compressors may use oil for different purposes,such as to lubricate components of the compressor. The oil can mix withthe working fluid and leave the compressor, which can affect theoperation of the compressor and reduce the heat transfer and energyefficiency of the working fluid. The refrigeration system can usedifferent piping configurations to return the oil to the compressor.Methods and equipment for returning the oil to the compressor aresought.

SUMMARY

Implementations of the present disclosure include a method that includesreceiving, from a variable frequency drive of one or more compressors ofa refrigeration system and by a processing device, operation informationof the one or more compressors. The method also includes comparing, bythe processing device, the operation information of the one or morecompressors to an operation threshold. The method also includesdetermining, by the processing device and based on the comparison, thatthe operation information of the one or more compressors satisfies theoperation threshold. The method also includes changing, based on thedetermination that the operation information of the one or morecompressors satisfies the operation threshold and by the processingdevice, an operation parameter of a component of the refrigerationsystem. Changing the operation parameter increases at least one of: (i)a velocity of a working fluid in a piping assembly fluidly coupled tothe one or more compressors, or (ii) a flow rate of an oil in the pipingassembly flowing into the one or more compressors.

In some implementations, the operation information includes a frequencyof the one or more compressor, and changing the operation parameterincludes changing an operation speed of the one or more compressors. Insome implementations, the frequency includes a frequency of the one ormore compressors over a period of time and the operation thresholdincludes a frequency threshold. The method further includes, beforecomparing the operation information to the operation thresholddetermining an average frequency of the compressor over a predeterminedperiod of time, and determining that the operation information satisfiesthe operation threshold includes determining that the average frequencyof the compressor is below the frequency threshold.

In some implementations, the piping assembly includes a suction linefluidly coupled to a fluid inlet of the one or more compressors. Themethod further includes, after changing the operation parameter,receiving, by the processing device and from one or more sensors fluidlycoupled to the suction line, fluid information including a parameter ofthe working fluid in the suction line. The method also includescomparing, by the processing device, the parameter of the working fluidto a fluid parameter threshold. The method also includes determining, bythe processing device and based on the comparison, that the parameter ofthe working fluid satisfies the fluid parameter threshold. The methodalso includes opening, by the processing device and based on thedetermination that the parameter of the working fluid satisfies thefluid parameter threshold, at least one of (i) a liquid injection valvein fluid communication with the suction line, or (ii) a gas injectionvalve in fluid communication with the suction line, chancing atemperature of the working fluid in a superheated state. In someimplementations, the parameter of the working fluid includes atemperature or a pressure of the working fluid in the suction line, andthe fluid parameter threshold includes (i) a respective temperaturethreshold of the working fluid that indicates a low load condition ofthe refrigeration system or (ii) a pressure threshold of the workingfluid that indicates a low load conditions of the refrigeration system.Determining that the parameter of the working fluid satisfies the fluidparameter threshold includes determining that the parameter of theworking fluid is at or below the fluid parameter threshold.

In some implementations, the piping assembly includes a suction linefluidly coupled to a fluid inlet of the one or more compressors. Themethod further includes, after changing the operation parameter,receiving, by the processing device and from one or more sensors fluidlycoupled to the suction line, fluid information including a parameter ofthe working fluid in the suction line. The method also includescomparing, by the processing device, the parameter of the working fluidto a fluid parameter threshold. The method also includes determining, bythe processing device and based on the comparison, that the parameter ofthe working fluid does not satisfy the fluid parameter threshold. Themethod also includes, upon determining that the parameter of the workingfluid does not satisfy the fluid parameter threshold, repeating thesteps of claim 1.

In some implementations, the piping assembly includes a suction linefluidly coupled to the one or more compressor, and changing theoperation parameter includes changing a pressure set point of at leastone of a flash tank of the refrigeration system or a gas cooler of therefrigeration system, increasing a speed of the working fluid flowing inthe suction line.

In some implementations, further including, after changing the operationparameter, receiving, by the processing device and from one or moresensors of the refrigeration system, a discharge air temperature of anevaporator of the refrigeration system. The method also includescomparing, by the processing device, the discharge air temperature to adischarge air temperature threshold. The method also includesdetermining, by the processing device and based on the comparison, thatthe discharge air temperature satisfies the discharge air temperaturethreshold. The method also includes, upon determining that the parameterof the fluid satisfies the fluid parameter threshold, repeating thesteps of claim 1.

In some implementations, the method also includes, after changing theoperation parameter, receiving, by the processing device and from one ormore sensors of the refrigeration system, a discharge air temperature ofan evaporator of the refrigeration system. The method also includescomparing, by the processing device, the discharge air temperature to adischarge air temperature threshold. The method also includesdetermining, by the processing device and based on the comparison, thatthe discharge air temperature does not satisfy the discharge airtemperature threshold. The method also includes resetting, by theprocessing device, a timer of the refrigeration system, the processingdevice configured to repeat the steps of claim 1 after a predeterminedperiod of time indicated by the timer.

In some implementations, changing the pressure set point includeslowering a pressure set point of the flash tank or increasing a pressureset point of the gas cooler, increasing a speed of the working in thesuction line.

In some implementations, the piping assembly includes a suction linefluidly coupled to the one or more compressor, and changing theoperation parameter includes changing a suction set point of therefrigeration system, increasing a speed of the working fluid flowing inthe suction line.

In some implementations, increasing the velocity of the working fluidincludes increasing a flow rate of oil return into the one or morecompressors.

In some implementations, the piping assembly extends from one or moreevaporators of the refrigeration system to the one or more compressors,and changing the operation parameter includes changing the operationparameter to increase the velocity of the working fluid in the pipingassembly flowing from the one or more evaporators to the one or morecompressors.

In some implementations, the piping assembly includes a low temperaturesuction line and a medium temperature suction line. The one or morecompressors includes a group of low temperature compressors and a groupof medium temperature compressors, and the one or more evaporatorsincludes a group of low temperature display cases fluidly coupled,through the low temperature suction line, to the group of lowtemperature compressors and a group of medium temperature display casesfluidly coupled, through the medium temperature suction line, to themedium temperature compressors, and changing the operation parameterincludes changing the operation parameter to increase the velocity ofthe working fluid in at least one of the low temperature suction line ofthe medium temperature suction line.

In some implementations, the low temperature suction line includes aheat exchanger coil disposed inside the flash tank. The low temperaturesuction line includes a uniform diameter, and changing the operationparameter includes changing the operation parameter to increase thevelocity of the working fluid flowing in the low temperature suctionfrom the heat exchanger coil to the group of low temperaturecompressors.

In some implementations, the operation information includes an oil levelin an oil separator fluidly coupled to the one or more compressors, andthe operation threshold includes an oil level of the oil separator underlow load conditions of the refrigeration system.

Implementations of the present disclosure include a method that includesobtaining a refrigeration system including one or more compressors, oneor more evaporators, a piping assembly fluidly coupled to andinterconnecting the one or more compressors to the one or moreevaporators, and a working fluid configured to flow in the pipingassembly from the one or more evaporators to the one or morecompressors. The method also includes changing, based on an indicationof reduced velocity of the working fluid along the piping assembly, anoperation parameter of a component of the refrigeration system, therebyincreasing at least one of: (i) a velocity of a working fluid in apiping assembly fluidly coupled to the one or more compressors, or (ii)a flow rate of an oil in the piping assembly flowing into the one ormore compressors.

In some implementations, the one or more compressors includes a group ofcompressors and changing the operation parameter of the component of therefrigeration system includes at least one of: (i) increasing afrequency of a lead compressor of the group of compressors, (ii)lowering a pressure set point of a flash tank of the refrigerationsystem, (iii) increasing a pressure set point of a gas cooler of therefrigeration system, or (iv) lowering a suction pressure or temperatureset point of the refrigeration system.

In some implementations, the piping assembly includes a suction linefluidly coupled to a fluid inlet of the one or more compressors. Themethod further includes, after changing the operation parameter:receiving, by the processing device and from one or more sensors fluidlycoupled to the suction line, fluid information including a temperatureor pressure of the working fluid in the suction line near the fluidinlet of the one or more compressors. The method also includescomparing, by the processing device, the parameter of the working fluidto a fluid parameter threshold. Then method also includes determining,by the processing device and based on the comparison, that the parameterof the working fluid satisfies the fluid parameter threshold. The methodalso includes opening, by the processing device and based on thedetermination that the parameter of the working fluid satisfies thefluid parameter threshold, at least one of (i) a liquid injection valvein fluid communication with the suction line, or (ii) a gas injectionvalve in fluid communication with the suction line, chancing atemperature of the working fluid in a superheated state.

Implementations of the present disclosure include a system that includesat least one processing device a memory communicatively coupled to theat least one processing device. The memory stores instructions which,when executed, cause the at least one processing device to performoperations that includes receiving, by the at least one processingdevice and from one or more sensors or a VFD of a refrigeration system,operation information of one or more compressors of the refrigerationsystem. The operations also include comparing, by the processing device,the operation information of the one or more compressors to an operationthreshold. The operations also include determining, by the processingdevice and based on the comparison, that the operation information ofthe one or more compressors satisfies the operation threshold. Theoperations also include changing, based on the determination that theoperation information of the one or more compressors satisfies theoperation threshold and by the processing device, an operation parameterof a component of the refrigeration system, increasing at least one of:(i) a velocity of a working fluid in a piping assembly fluidly coupledto the one or more compressors, or (ii) a flow rate of an oil in thepiping assembly flowing into the one or more compressors.

Particular implementations of the subject matter described in thisspecification can be implemented so as to realize one or more of thefollowing advantages. For example, the refrigeration system of thepresent disclosure can increase the flow rate of the oil to thecompressor while simplifying the piping design and assembly. Therefrigeration control system of the present disclosure can allow thepiping assembly to have a common line size or a small number of linesizes for a wide range of capacity points, which can reduce design,manufacturing, and installation costs.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a refrigeration system according toimplementations of the present disclosure.

FIG. 2 is a block diagram of a control system for the refrigerationsystem according to implementations of the present disclosure.

FIG. 3 is a decision flow diagram of a control system for therefrigeration system according to a first implementation of the presentdisclosure.

FIG. 4 a decision flow diagram of a control system for the refrigerationsystem according to a second implementation of the present disclosure.

FIG. 5 is a flow diagram of an example method of increasing a velocityof a working fluid in a piping assembly according to a firstimplementation of the present disclosure.

FIG. 6 is a flow diagram of an example method of increasing a velocityof a working fluid in a piping assembly according to a secondimplementation of the present disclosure.

FIG. 7 is a schematic diagram of a computer system that may be appliedto any of the computer-implemented methods and other techniquesdescribed herein.

DETAILED DESCRIPTION OF THE DISCLOSURE

Oil logging in the suction lines of a refrigeration system may happenduring low-load operating conditions (e.g., during winter months and atnight). To reduce or prevent oil logging in the suction lines andsimplify the piping design, a refrigeration system with demand oilreturn logic and with generally uniform line sizing can be implemented.

FIG. 1 shows a schematic diagram (e.g., a piping and instrumentationdiagram) of a refrigeration system or assembly 100. The refrigerationsystem 100 can be e.g., a commercial CO2 refrigeration system, anammonia refrigeration system, or a chilled water refrigeration system.The refrigeration system 100 can be a basic system with one compressoror a group of compressors (e.g., transcritical compressors, subcriticalcompressors, or a combination of the two). The refrigeration system 100can include one evaporator or a group of evaporators (e.g.,medium-temperature display cases, low-temperature display cases, or acombination of the two).

As shown in FIG. 1 , the refrigeration system 100 can include two typesof compressors or groups of compressors and two types of evaporators orgroups of evaporators. For example, the refrigeration system 100 caninclude two groups of compressors and two groups of evaporators.Specifically, the refrigeration system 100 includes a first compressoror group of compressors 102 a (e.g., one or more transcriticalcompressors or medium temperature compressors) and a second compressoror group of compressors 102 b (e.g., one or more subcritical compressorsor low temperature compressors). The refrigeration system 100 alsoincludes a first evaporator or group of evaporators 108 a, (e.g., one ormore medium temperature evaporators), and a second evaporator or groupof evaporators 108 b (e.g., one or more low temperature evaporators).The medium-temperature evaporators 108 a can include, for example,refrigerated display cases that display medium-temperature merchandisesuch as non-frozen products, and the low-temperature display cases 108 bcan include, for example, refrigerated display cases that displaylow-temperature merchandise such as frozen products.

The refrigeration system 100 can also include a refrigeration controlsystem 101, a gas cooler or condenser 104, a receiver tank 106 (e.g., aflash tank or refrigerant liquid vapor separator), one or more oilseparators 103, one or more accumulators 129, and multiple fluid linesand valves fluidly connected to the respective equipment of therefrigeration system 100. The refrigeration system 100 also includessensors and one or more processing devices 120 and 121 communicativelyor electrically coupled to respective equipment of the refrigerationsystem 100.

Both groups of evaporators 108 a and 108 b are fluidly connected to theflash tank 106 to receive a portion of the working fluid from the flashtank 106. The low-temperature suction line 107 b of the low-temperatureevaporators 108 b is connected to the flash tank 106 to send fluid tothe flash tank 106. The subcritical compressors 102 b receive a vaporphase of the working fluid from the low-temperature evaporators 108 b.The transcritical compressors 102 a receive a vapor phase of the workingfluid from the medium-temperature evaporators 108 a and from thesubcritical compressors 102 b.

During an exemplary refrigeration cycle, medium-temperature dischargegas (or liquid and gas) flows from the gas cooler 104 to the receivertank 106. Before reaching the receiver tank 106, a high pressure controlvalve 140 can change (e.g., reduce) the pressure of themedium-temperature discharge gas (or high-pressure condensate) toincrease the amount of liquid phase of the working fluid. A firstportion of the liquid phase of the working fluid flows from the tank 106to the low-temperature evaporators 108 b, with the working fluid passingfirst through respective expansion valves. A second portion of theliquid phase of the working fluid flows from the tank 106 to themedium-temperature evaporators 108 a, with the working fluid passingfirst through respective expansion valves. After passing through thelow-temperature evaporators 108 b, the working fluid, as alow-temperature suction gas, flows through the low-temperature suctionline or lines 107 b to the receiver tank 106, and from the tank 106 tothe subcritical compressors 102 b. The suction line 107 b can be fluidlycoupled to the accumulator 129 that can meter or prevent or decrease theflow of fluid refrigerant and oil back to the compressors 102 b. Theworking fluid, as a low-temperature discharge gas, flows from thesubcritical compressors 102 b to mix with the medium temperature suctiongas that flows from the medium-temperature evaporators 108 a to thetranscritical compressors 102 a. The medium temperature suction gasflows through a medium temperature suction line 107 a to thetranscritical compressors 102 a. To finish the cycle, medium temperaturedischarge gas flows from the transcritical compressors 102 a to the gascooler 104.

The oil separators 103 can separate some or all the oil from therefrigerant to flow the oil back to the transcritical compressors 102 aand subcritical compressors 102 b. The rest of the oil can flow to thesuction lines 107 b and 107 a, and the processing devices 120 and 121can control different equipment of the system 100 to flow such oil backto the compressors 102 a and 102 b.

The flash tank 106 separates a liquid phase of the working fluid from avapor phase of the working fluid. The flash tank 106 can store a portionof the liquid phase and supply the rest to the evaporators 108 a and 108b. The receiver tank 106 supplies the separated vapor (e.g., flash gas)to the medium temperature suction line 107 a of the transcriticalcompressors 102 a. The separated vapor can pass through a pressurecontrol or regulating valve 142 (e.g., a flash gas bypass valve) thatcan change the pressure of the vapor. Additionally, the pressure controlvalve 142 can help regulate the pressure of the flash tank 106.

A pressure control valve or regulating valve 140 (e.g., a high pressurecontrol valve) can regulate the pressure of the gas cooler 104. Bothvalves 140 and 142 can together control the pressure of the flash tank106. The compressed working fluid from the subcritical compressors 102 bis fed to the medium temperature compressor suction line 107 a. Themedium temperature suction gas from the medium temperature display cases108 a mixes with the flash gas and those gases mix with the compressedworking fluid from the subcritical compressors 102 b. Such mixture isflown to the transcritical compressors 102 a to begin a new cycle.

To prevent or reduce a low super heat in the suction lines 107 a and 107b, hot gas can be injected form a medium temperature discharge to themedium temperature suction line 107 a and to the low temperature suctionline 107 b. To prevent or reduce a high superheat in the suction lines,liquid refrigerant can be injected from a liquid supply to the mediumtemperature suction line 107 a and to the low temperature suction line107 b.

The low temperature suction line 107 b can include a supply line 214that supplies the working fluid to the receiver tank 106 and a returnline 218 that returns or flows the working fluid from the receiver tank106 to the subcritical compressors 102 b. The low temperature suctionline 107 b can include or be fluidly connected to a heat exchanger 200disposed inside the receiver tank 106. The working fluid inside the heatexchanger 200 transfers heat through a heat transfer surface (e.g., acoil surface) of the heat exchanger 200 to the liquid or condensateinside the receiver tank 106. In some implementations, the mediumtemperature suction line 107 a can have or be connected to a similarheat exchanged inside the receiver tank 106.

The oil separators 103 can help convey oil back to the compressors 102,but the oil that escapes the separators 103 can accumulate in thesuction lines 107 b and 107 a during low load conditions of the system100. The heat exchanger 200 inside the flash tank 106 can include doublerisers and P-traps to help flow the oil back to the compressors. Inaddition to or instead of relying on the heat exchanger to return theoil back to the compressors, a computer-implemented control system 101can be used to flow the oil back to the compressors. As furtherdescribed in detail below with respect to FIGS. 3-4 , control system 101uses the processing devices 120 and 121 to control the equipment of therefrigeration system 100 to flow the oil back to the compressors.

The working fluid may include a mixture of refrigerant and oil that,during low-load conditions, may leave behind the oil which thenaccumulates along the tubing (e.g., due to the relatively low velocityof the refrigerant). The refrigeration system 100 can be considered torun at low-load conditions when the system operates at about 5% to 20%of the total load capacity (e.g., rated load). For example, if therefrigeration system 100 is designed to remove the heat load of 100,000BTUs per hour (BTUH), then the system is considered as running under lowload conditions from about 5,000 BTUH to 20,000 BTUH. Such conditionscan be experienced, for example, during winter days and during times oflow foot traffic (e.g., during night time). During this time, not allcompressors may run and one compressor may run at low speed. Low loadconditions causes the velocity in the suction lines 107 a and 107 b todecrease. During such conditions, carried over oil can be stagnant inthe evaporator coil over long periods of time (e.g., 24 hour or more).At this time, the processing device can use demand oil return controllogic to forcefully pull the oil from the evaporator coils and flow theoil back to the compressors. In some implementations, the use of controllogic allows the suction lines 107 a and 17 b to have a uniform diameteror fewer changes in diameter than other refrigeration systems (e.g., twodifferent diameters).

During low load conditions, the vapor phase of the working fluid can berunning at a speed at which the velocity of the vapor in the pipe is toolow to maintain the oil entrained to return the oil back to thecompressor. This may result in oil deposits or clogs within therefrigeration system piping or components. To help return the oil backto the compressor 102 a or 102 b, the refrigeration control system 101increase the speed of the refrigerant to have enough friction with theoil to return the oil back to the compressor. To increase the velocityof the refrigerant, the processing device 120 can selectively control orchange operating parameters of different components of the refrigerationsystem 100.

The oil that leaves the compressors 102 b and 102 a can be lubricationoil from the crankcase of the compressors. Because some refrigerantssuch as CO₂ are miscible with oil, the refrigerant mix with thelubrication oil as the refrigerant flows through the compressors. Thus,the compressors can discharge the high pressure and high temperatureworking fluid with some carried over oil (e.g., about 0.2% to 5% of oilis carried over of total mass flow). During normal or full loadoperation, the carried over oil often returns to the compressors throughthe suction lines. A velocity range for the discharge line during normaloperation can be between about 1000 FPM to 2500 FPM. For example, avelocity range for the medium temperature suction line 107 a duringnormal operation can be between about 700 FPM to 2500 FPM, and avelocity range for the low temperature discharge line 107 b duringnormal operation can be between about 1000 FPM to 2500 FPM. During lowload operation, the velocities can be lower than the normal velocities,which causes the oil to stay in the evaporator coil.

The refrigeration control system 101 includes one or more processingdevices 120 and 121 (e.g., computer processors with controllers),multiple sensors or groups of sensors 125, 127, 128, and 146, and atriggering mechanism 126, such as an algorithm or program or softwareimplemented with the processing devices. The triggering mechanism can bea timer, a clock, a thermostat, or any device or algorithm that cantrigger the processing device based on time intervals, predeterminedtimes of the day or seasons, ambient or indoor temperatures, and otherrelated parameters.

The sensors 125, 127, 128, and 146 can include, without limitation,pressure sensors (e.g., pressure transducers), temperature sensors, oillevel sensors, humidity sensors, and vibration sensors. For example, afirst group of pressure and temperature sensors 127 is coupled to themedium temperature suction line 107 a, a second group of pressure andtemperature sensors 128 is coupled to the low temperature suction line107 b, and a third group of pressure and temperature sensors 127 iscoupled to the discharge gas line that connects the subcriticalcompressors 102 b to the medium temperature suction line 107 a. Oillevel sensors 146 with or without solenoid valves can be coupled to eachcompressor to detect a level of oil within each compressor.

In some implementations, the processing devices 120 and 121 can beimplemented as one or more processors, computers, microcontrollers, or acombination thereof. For example, the first processing device 120 caninclude a rack controller and the second processing device 121 caninclude a case controller. The processing devices 120 and 121 can bepart of a single or separate electrical control panels. In someimplementations, the processing devices can be implemented as adistributed computer system disposed partly at the compressors (or someother equipment of the system) and partly at an electrical controlpanel. The computer system can include one or more processors and acomputer-readable medium storing instructions executable by the one ormore processors to perform the operations described here. In someimplementations, the processing devices 120 and 121 can be implementedas processing circuitry, firmware, software, or combinations of them.The processing devices 120 and 121 can include a building management orautomation system (BMS). The processing devices 120 and 121 can transmitsignals and control the multiple components of the refrigeration system100 and control system 101 to change a velocity of the working fluidalong the piping assembly 107.

The second processing device 121 can control the expansion valves of theevaporators 108 a and 108 b and the first processing device 120 cancontrol a hot gas injection valve 132 connected to a line extending fromor near an inlet of the gas cooler 104 to the low temperature suctionline 107 b. The first processing device 120 can control the rest of thevalves, sensors, gas cooler, variable frequency drives (VFDs) of thecompressors, and other equipment of the refrigeration system 100.Alternatively, the system can only include one processing device 120that controls all of the components of the refrigeration system 100.

In some implementations, the refrigeration system 100 also includesliquid injection valves 130 and a gas injection valves 132. The liquidinjection valves 130 and a gas injection valves 132 can be used tochange a temperature of the suction lines 107 a and 107 b. For example,the processing device 120 can calculate a super heat from pressure andtemperature information received from the sensors 127 and 128. If suchtemperature satisfies a high super heat temperature threshold, theprocessing device 120 can open one or both of the liquid injectionvalves 130 to inject liquid when the super heat is high in the suctionlines. Conversely, if such temperature satisfies a low super heattemperature threshold, the processing device 120 can open one or both ofthe hot gas injection valves 132 to inject hot gas when the super heatis low in the suction lines.

Additionally, the processing device 120 can control the liquid injectionvalves 130 and the gas injection valves 132 based on feedback from thecompressors 102 a and 102 b, which may include speed and timing of thecompressors. For example, if one or more of the compressors are runningat low speed (e.g., about 30 Hz for more than 24 hours), there is apossibility of oil logging in the evaporator coil. Upon determining thatthe compressors have been running at or below a predetermined frequencyfor a predetermined period of time, the processing device 120 cancontrol a respective VFD 160 associated with the desired compressor orgroup of compressors to increases the compressor's speed. The incrementof the compressor's speed can bring the logged oil back to thecompressor.

FIG. 2 shows a block diagram of the refrigeration control system 101.The processing device 120 (e.g., the first processing device 120 in FIG.1 or a combination of the first and second processing devices 120 and121) receive inputs from sensors 125, 127, 128, and 146 to control,based on the input from the sensors, at least one of the liquidinjection valves 130, the gas injection valves 132, the pressure controlvalves 140, the bypass valves 142, the gas cooler 104, the VFDs 160, orother components of the refrigeration system 100 to increase the speedof the working fluid.

The processing device 120 can be communicatively coupled to a memory 150that stores instructions that the processing device 120 can execute tomake determinations, based on the information received from the sensoror other components, to return the oil back to the compressor 102 a or102 b. For example, the processing device 120 can compare theinformation received from the sensors or other components to a threshold(e.g., an operation threshold) to determine if the processing device 120needs to increase the speed of the working fluid. The differentparameters can represent information such as operating parameters of thecompressor, operating parameters of the gas cooler 104, operatingparameters of the flash tank 106, operating parameters of the valves, oroperating parameters of other components of the system. The processingdevice 120 changes, based on the comparison, one or more operatingparameter of one or more component of the refrigeration system 100 toincrease the speed of the working fluid.

For example, the processing device 120 can increase an operating speedof the compressors 102 a and 102 b or lower a set point (e.g., atemperature set point or a suction pressure set point) of therefrigeration system, or a pressure set point of the flash tank 106 orthe gas cooler 104. An exemplary set point may range from 5° F. to 30°F. for the medium temperature suction group and −5° F. to −30° F. forthe low temperature suction group. For example, if the mediumtemperature suction is set at 15° F., the processing device 120 canlower the medium temperature suction to 5° F. during the low loadcondition, which can increase the refrigeration demand and force thecompressor to run at full speed. If the compressors are forced to run atfull speed, the suction flow rate can increase, which helps return theoil from evaporator to compressors.

In an exemplary implementation, the processing device 120 can set themedium temperature suction group to run at 15° F. (based on thetemperature set point), with the corresponding suction pressure being at375.8 PSIG. Either temperature or pressure can be used as the set point.The processing device 120 or an operator can run the system at such setpoint for a set period of set time (e.g., 24 hours), during which therefrigeration system 100 is running at low speed. The processing device120 can lower, based on determining that the system is running at lowspeed, the suction set point to 13° F. Based on the change in demand,the processing device 120 controls the compressors to run and reduce thetemperature and pressure to 13° F. and 363.3 PSIG respectively.

In another exemplary implementation, the processing device 120 canlower, based on a predetermined load condition, the flash tank set pointfrom 36.5° F./528 PSIG to 33° F./498.5 PSIG. The pressure in the suctionlines is lower than the pressure of the flash tank 106. To lower thepressure of the flash tank 106, the processing device 120 can open theflash gas bypass valve 142 to flow the fluid from the flash tank to thesuction line 107 a, lowering the flash tank pressure and increasing thesuction pressure. With the suction pressure higher, the compressors areforced to run at full speed (or increased speed) to bring down thesuction pressure to the suction set point.

In another exemplary implementation, the processing device 120 canincrease, based on a predetermined load condition, the gas coolerpressure set point from 1105 PSIG to 1155 PSIG. The processing device120 increases the pressure of the gas cooler 104 by opening or closingthe high pressure valve 140 and by controlling the speed of thecompressors and of the gas cooler fans. The gas cooler pressure setpoint can be variable based on the ambient temperature. The gas coolerpressure set point can be is controlled in a subcritical operatingcondition and in a transcritical operating condition. In the subcriticalcondition, pressure and temperature are related and controlled together,and in the transcritical condition, pressure and temperature are notrelated and are controlled independently. Increasing the gas coolerpressure set point forces the gas cooler fan to turn off or slow down toget to 1155 PSIG and can force the compressors to run to increase thegas cooler pressure. After the pressure setpoint of the gas cooler 104increases, the processing device 120 can set the system back to a normalor standard set point. With the pressure of the gas cooler 104 high, thehigh-pressure valve 140 opens, which increases the pressure of flashtank 106. The increase in pressure of the gas in the flash tank causesthe bypass valve 142 to open, increasing the suction pressure andforcing the compressors to run faster.

The processing device 120 can control the compressors 102 a and 102 bbased on information received from the VFDs 160, from the compressors102 a and 102 b, or from any of the sensors 125, 127, 128, and 146. Forexample, the processing device 120 can receive (e.g., in real time ornear real time), from the VFDs 160 (or directly from a motor or anothercomponent of the compressor 102 a and 102 b), operation information suchas the operating frequency or power of the compressor 102 a and 102 b.By determining the operating speed or frequency of the compressors, theprocessing device 120 can determine that the refrigerant velocity is toolow and can then control components of the system to increase its speed.Additionally, the operation information can include information thatrepresents how many compressors of a group of compressors are running.If a small enough number of compressors is running, the processingdevice 120 can determine that the refrigerant velocity is too low andcan then control components of the system to increase the speed of theworking fluid.

As used herein, the term “real-time” refers to transmitting orprocessing data without intentional delay given the processinglimitations of a system, the time required to accurately obtain data,and the rate of change of the data. Although there may be some actualdelays, the delays are generally imperceptible to a user.

Refrigerant velocity (and thus load conditions) can additionally bedetermined by other components such as the opening percentage of highpressure control, the opening percentage of the flash gas bypass controlvalve, number of expansion valves open at the display cases orevaporators, or electric current drawn at the rack by the compressors.

As shown in FIG. 1 , in some cases, only a lead of a group ofcompressors 102 a and 102 b is connected to the VFD 160 (or to digitalunloaders). The processing device 120 can send instructions to the VFD160 to change a speed of the compressors. For example, the VFD 160 canchange the speed of the lead compressor from 25 Hz to 75 Hz in bothsuction groups 102 a and 102 b. Other compressors may or may not havevariable frequency drives to control the compressor speeds, but theprocessing device 120 can turn on or off those compressors based on therequired load. Another parameter that the processing device 120 canchange includes digital unloaders, which can change the compressorcapacities from 10% to 100%.

Upon determining the operating frequency (e.g., the average operatingfrequency), the processing device 120 can compare the average operatingfrequency to an operation threshold. For example, the operationthreshold can include an operating frequency or power of the compressor102 a or 102 b. The operating frequency threshold can be a frequency ofthe compressor under normal or high load conditions. For example, thefrequency threshold can include an average frequency of the compressorduring normal business hours (e.g., during the day, and during summermonths) when the refrigeration system works at a speed at which the oilreturns normally to the compressor. In some implementations, theoperating information can include an oil level in a reservoir of thecompressor.

The group of oil separators 103 are equipped with oil level sensors. Thegroup of oil separators 103 can include an oil separator and an oilreservoir separate from the oil separator. When the separator separatesthe oil from the discharge gas and oil level reaches the level sensor,the level sensor sends signal to the solenoid valve residing between theoil separator and oil reservoir. The signal is a signal to open and feedof the oil reservoir. The oil level sensor in the oil reservoir can alsoprovide signal to the processing device 120 if there is low oil level inthe reservoir. The compressors 102 a and 102 b can be equipped with oillevel sensors 146 and with respective solenoid valves. If the oil levelof a compressor is low, the processing device 120 activates thereservoir (e.g., the solenoid valve of the oil reservoir) to feed thecompressors from the reservoir.

FIG. 3 shows a decision flow diagram 300 of the refrigeration controlsystem 101. For example, the processing device 120 can control therefrigeration system 100 based on inputs and decisions made with respectto those inputs. In the first step 301, the refrigeration control system101 starts by receiving input from the sensors to control components ofthe refrigeration system based on the sensor input. In step two 302, theprocessing device 120 calculates an average frequency of the compressor(e.g., a lead compressor). In some implementations, instead of or inaddition to calculating the average frequency of the compressor 102 a or102 b, the processing device 120 determines an oil level of the oilreservoir. In step three 304, the processing device 120 determines ifthe average frequency satisfies a threshold (e.g., an operatingfrequency threshold). The threshold can be a low operating frequencysuch as a frequency during low load conditions of the system 100.Satisfying the threshold can include operating at a frequency that is ator below the threshold.

In some implementations, instead of or in addition to determining if theaverage frequency satisfies the threshold, the processing device 120 candetermine if the oil level of the oil reservoir satisfies an oil levelthreshold. The oil level threshold can be a low oil level that indicatesthat the refrigeration system is under low load conditions or that thecompressor 102 a or 102 b needs to increase its oil level to operatemore efficiently. Satisfying the oil level threshold can include havingan oil level that is at or below the oil level threshold.

If the operating frequency threshold or the oil level threshold is notsatisfied, the processor can wait a predetermined period of time toagain calculate the average operating frequency of the compressor. Insome implementations, if the processing device 120 determines that theparameters do not meet the thresholds, the processing device 120 cantake step eight 314 by triggering mechanism (e.g., the timer). Forexample, the timer can cause the processing device 120 to begin theprocess again after a predetermined period of time. In step nine 316,the triggering mechanism can transmit or send input to the processingdevice 120 after a predetermined period of time to start calculating thefrequency of the compressor again.

The triggering mechanism can be configured to trigger the processingdevice during the times of the day or seasons of the year when therefrigeration system is expected to run at low-load conditions. Forexample, the triggering mechanism 126 can trigger or activate theprocessing device 120 during the night when outdoor temperaturesdecrease and the refrigerators or freezers remain closed, or duringwinter months when the outdoor temperatures are relatively low. Thetriggering mechanism 126 can be manually or automatically set up basedon room temperature information, weather information, or other relatedinformation.

If the processing device 120 determines that the operating frequencythreshold or the oil level threshold is satisfied, the processing device120 takes step four 306 by changing an operating frequency of acomponent of the refrigeration system 100 for a predetermined period oftime. For example, the processing device 120 can determine an operatingcommand and control, based on the operating command, an operatingparameter of a component of the refrigeration system 100.

The operating command can include instructions to lower a pressure setpoint or the suction set point of the system 100. For example, after theprocessing device 120 determines that the operation information of thecompressor 102 a or 102 b satisfies the threshold, the processor canlower the suction pressure set point or the pressure set point of theflash tank 106, or the gas cooler 104, or a combination of them. Thus,the operating command can include a set point operating command.

Referring back to FIG. 1 , the flash tank 106 can have a pressure setpoint that ranges from about 30 bar to 90 bar. The pressure set point ofthe flash tank 106 can be set, for example, at 35 bar. Based on theconfiguration of the refrigeration system 100, the processing device 120can lower the pressure set point of the flash tank 106 in one bar rangeor increments. For example, the processing device 120 can lower thepressure set point of the flash tank 106 from 34 bar to 33 bar, thenfrom 33 bar to 32 bar. The suction set point may range from 5° F. to 30°F. for the medium temperature suction group and −5° F. to −30° F. forthe low temperature suction group. For example, if the mediumtemperature suction is set at 15° F. for the application, once a lowload condition is detected, the medium temperature suction can be set to5° F., which increases the refrigeration demand and forces thecompressors to run at faster (e.g., at full speed). Once the oil levelreaches a normal level, the processing device 120 resents the pressureset point to normal operating condition.

Referring back to FIG. 3 , to determine if the change in pressure setpoint has helped return the desirable amount of oil back to thecompressors 102 a or 102 b, the system 100 can determine the oil levelsor measure a discharge air temperature. For example, in step five 308,the sensors 124 measure a fluid parameter (e.g., discharge airtemperature) of the evaporators 108 a or 108 b. The sensors transmit, tothe processing device 120, the fluid parameter at the evaporator 108 aor 108 b (or refrigerator display cases) of the refrigeration system.

The discharge air temperature of the evaporators 108 a or 108 b is thetemperature inside the evaporator or display case, or the temperature ofthe food inside the display case. For example, the evaporator cancontinuously discharge 15° F. air to maintain the food temperature at20° F. in the evaporator or display case. If during load conditions theair inside the display case is 15° F., the expansion valve at the inletof an evaporator coil can close to stop the working fluid fromcirculating in that particular display case. If this happens in themajority of display cases, the flow of refrigerant from the displaycases to the compressors is partial or low enough to meet a lowthreshold that indicates low load conditions.

In step six 310, the processing device 120 can compare the temperatureof the discharge air to a discharge air threshold. The threshold caninclude a temperature of the discharge air at the outlet of theevaporator under low load conditions, which indicates low velocity ofthe working fluid.

The processing device 120 can determine, based on the comparison, thatthe temperature of the discharge air satisfies the fluid parameterthreshold. For example, the temperature of the discharge air can be ator below the threshold. In such cases, the velocity of the working fluidmay be low enough to cause oil logging or accumulate in the pipingassembly. Upon determining that the parameter of the fluid satisfies thefluid parameter threshold, the processing device 120 can go back to steptwo 302 by calculating the average operating frequency of the compressor102 a or 102 b. Doing so can keep the suction pressure set point at thedesired level until the working fluid reaches a desirable velocity orthe oil reaches a desirable flow rate into the compressor.

If the processing device 120 can instead determines, based on thecomparison, that the temperature of the discharge air does not satisfythe fluid parameter threshold, the processing device can take step seven312 by resetting the set point. For example, the processing device 120can reset the suction pressure or temperature set point, the pressureset point of the flash tank 106, or the pressure set point of the gascooler 104. For example, the processing device 120 can determine a resetcommand and transmit the reset command to any of these components orvalves to reset the set point. The processing device 120 canadditionally reset the triggering mechanism to return the control system101 to a standard oil return logic.

FIG. 4 shows a decision flow diagram 400 of the refrigeration controlsystem 101 similar to the decision flow diagram of FIG. 3 , but used tocontrol operating parameters of the compressors 102 a or 102 b. In stepone 401, the processing device 120 starts the process by receivingsensor input. In step two 402, the processing device 120 calculates anaverage frequency of the compressor 102 a or 102 b. In step three 404,the processing device 120 then determines if the average frequencysatisfies a threshold (e.g., an operating frequency threshold). If theoperating frequency threshold (or the oil level threshold) is notsatisfied, the processor 120 takes step 8 by resetting the timer andwaiting a predetermined period of time (e.g., until the timer triggersthe processing device again) to again calculate the average operatingfrequency of the compressor. In step four 406, if the processing device120 determines that the operating frequency threshold is satisfied, theprocessing device 120 increases the operating frequency of thecompressor 102 a or 102 b. The frequency of the compressor can bechanged by the VFD. Increasing the operating frequency of the compressorcan increase the velocity of the working fluid.

In step five 408, to determine if the increase in operating frequency ofthe compressor 102 a or 102 b has helped return the desirable amount ofoil back to the compressor, the processing device 120 can receive, froma sensor 127 or 128 at or near the inlet of the compressor, fluidinformation that includes a parameter of a fluid at the inlet of thecompressor. The parameter of the fluid can be a temperature or pressureof the working fluid at the inlet of the compressor.

When the compressor 102 a or 102 b is forced to run at full speedwithout there being a load requirement from the display cases, thesuction pressure and temperature may go below the pre-defined setpoint/threshold, which can cause low superheat issues. In step four 406,the processing device 120 can force the compressor to run at full speedkeeping while monitoring the suction pressure and temperature (stepsfive and six 408 and 410). If the pressure or temperature meets thethreshold, the processing device 120 calculates the superheat todetermine if the superheat is low (e.g., below 10° F.). If super thesuper heat is low (e.g., meets a superheat temperature threshold), thenthe processing device 120 takes step seven 412 by triggering the hot gasinjection valve to open, which increases the super heat. The superheatis increased until the temperature is above the threshold, in which casethe processing device 120 goes back to step two 402.

In step eight 414, if the processing device 120 determines in step three404 that the parameters do not meet the threshold, the processing device120 resets the triggering mechanism. In step nine 416, the processingdevice determines if a pre-determined period of time has passed to beginthe process again.

The refrigeration control system 101 can operate using an intrusive or anon-intrusive method. For example, the intrusive method includesoperating, based on the predefined set points, the refrigeration system101 efficiently, in which case the system 100 may go into instable modefor a period of time (e.g., a few minutes). The non-intrusive methoduses the same logic or steps that the intrusive method uses, but theprocessing device 120 uses additional proportional-integral-derivative(PID) logic to predict the low load conditions prior to occurring. Whenthe processing device 120 predicts that the low load condition mayoccur, the processing device 120 can run the compressor 102 a or 102 bat full speed to return any oil logged in the evaporator coil.Additionally, the processing device 120 can, to conserve energy orotherwise increase efficiency, predict when a low load condition will beshort enough to not require changing parameters of the system 100. Forexample, if the refrigeration system 100 is running at low loadcondition for 6 hours (e.g., during the night) and the processing devicedetermines that load requirements will increase shortly (e.g., due tothe store opening in the morning), the processing device 120 can wait apredetermined time period until the capacity requirement goes, forcingthe compressor to run at full speed and in-turn return the oil back tothe compressor without affecting the other parameters.

The methods and systems described herein help increase a velocity of theworking fluid in the suction lines 107 a or 107 b (or other lines of thesystem 100). The increase in speed of the working fluid flows the oilback to the compressors 102 a or 102 b. Because the processing device120 can help change the speed the working fluid in the piping assembly,the piping assembly (e.g., the suction lines 107 a and 107 b) can havepipes of a substantially equal diameter. The sizing of pipes can thus be“standardized” because the speed of the working fluid can becontinuously changed by the processing device 120, instead of dependingon the sizing of the pipes to achieve a desirable velocity of theworking fluid.

FIG. 5 shows a flow diagram of an example method 500 of returning oillogged in the fluid lines of a refrigeration system (e.g., therefrigeration system 100 in FIG. 1 ). The method includes receiving,from a variable frequency drive of one or more compressors of arefrigeration system and by a processing device, operation informationof the one or more compressors (505). The method also includescomparing, by the processing device, the operation information of theone or more compressors to an operation threshold (510). The method alsoincludes determining, by the processing device and based on thecomparison, that the operation information of the one or morecompressors satisfies the operation threshold (515). The method alsoincludes changing, based on the determination that the operationinformation of the one or more compressors satisfies the operationthreshold and by the processing device, an operation parameter of acomponent of the refrigeration system, increasing at least one of: (i) avelocity of a working fluid in a piping assembly fluidly coupled to theone or more compressors, or (ii) a flow rate of an oil in the pipingassembly flowing into the one or more compressors (520).

FIG. 6 shows a flow diagram of a second example method 600 of retuningcompressor oil to the compressor of a refrigeration system (e.g., therefrigeration system 100 in FIG. 1 ). The method includes obtaining arefrigeration system that includes one or more compressors, one or moreevaporators, a piping assembly, and a working fluid (605). The methodalso includes changing, based on an indication of reduced velocity ofthe working fluid along the piping assembly, an operation parameter of acomponent of the refrigeration system. The change causes an increase ofat least one of (i) a velocity of a working fluid in a piping assemblyfluidly coupled to the one or more compressors, or (ii) a flow rate ofan oil in the piping assembly flowing into the one or more compressors(610).

FIG. 7 is a schematic illustration of an example processing device orcontroller according to the present disclosure. For example, thecontroller 700 may include or be part of the processing device 120 shownin FIGS. 1-2 . The controller 700 is intended to include various formsof digital computers, such as printed circuit boards (PCB), processors,digital circuitry, or otherwise. Additionally the system can includeportable storage media, such as, Universal Serial Bus (USB) flashdrives. For example, the USB flash drives may store operating systemsand other applications. The USB flash drives can include input/outputcomponents, such as a wireless transmitter or USB connector that may beinserted into a USB port of another computing device.

The controller 700 includes a processor 710, a memory 720, a storagedevice 730, and an input/output device 740. Each of the components 710,720, 730, and 740 are interconnected using a system bus 750. Theprocessor 710 may include or be part of the processing device 120 shownin FIGS. 1-4 , and is capable of processing instructions for executionwithin the controller 700. The processor may be designed using any of anumber of architectures. For example, the processor 710 may be a CISC(Complex Instruction Set Computers) processor, a RISC (ReducedInstruction Set Computer) processor, or a MISC (Minimal Instruction SetComputer) processor.

In one implementation, the processor 710 is a single-threaded processoror microprocessor or parametric controller. In another implementation,the processor 710 is a multi-threaded processor. The processor 710 iscapable of processing instructions stored in the memory 720 or on thestorage device 730 to display graphical information for a user interfaceon the input/output device 740.

The memory 720 stores information within the controller 700. In oneimplementation, the memory 720 is a computer-readable medium. In oneimplementation, the memory 720 is a volatile memory unit. In anotherimplementation, the memory 720 is a non-volatile memory unit.

The storage device 730 is capable of providing mass storage for thecontroller 700. In one implementation, the storage device 730 is acomputer-readable medium. In various different implementations, thestorage device 730 may be a floppy disk device, a hard disk device, anoptical disk device, or a tape device.

The input/output device 740 provides input/output operations for thecontroller 700. In one implementation, the input/output device 740includes a keyboard and/or pointing device. In another implementation,the input/output device 740 includes a display unit for displayinggraphical user interfaces (e.g., a hand held device).

Although the following detailed description contains many specificdetails for purposes of illustration, it is understood that one ofordinary skill in the art will appreciate that many examples, variationsand alterations to the following details are within the scope and spiritof the disclosure. Accordingly, the exemplary implementations describedin the present disclosure and provided in the appended figures are setforth without any loss of generality, and without imposing limitationson the claimed implementations.

Although the present implementations have been described in detail, itshould be understood that various changes, substitutions, andalterations can be made hereupon without departing from the principleand scope of the disclosure. Accordingly, the scope of the presentdisclosure should be determined by the following claims and theirappropriate legal equivalents.

The singular forms “a”, “an” and “the” include plural referents, unlessthe context clearly dictates otherwise.

As used herein, the terms “aligned,” “substantially aligned,”“parallel,” or “substantially parallel” refer to a relation between twoelements (e.g., lines, axes, planes, surfaces, or components) as beingoriented generally along the same direction within acceptableengineering, machining, drawing measurement, or part size tolerancessuch that the elements do not intersect or intersect at a minimal angle.For example, two surfaces can be considered aligned with each other ifsurfaces extend along the same general direction of a device orcomponent.

As used in the present disclosure and in the appended claims, the words“comprise,” “has,” and “include” and all grammatical variations thereofare each intended to have an open, non-limiting meaning that does notexclude additional elements or steps.

As used in the present disclosure, terms such as “first” and “second”are arbitrarily assigned and are merely intended to differentiatebetween two or more components of an apparatus. It is to be understoodthat the words “first” and “second” serve no other purpose and are notpart of the name or description of the component, nor do theynecessarily define a relative location or position of the component.Furthermore, it is to be understood that that the mere use of the term“first” and “second” does not require that there be any “third”component, although that possibility is contemplated under the scope ofthe present disclosure.

What is claimed is:
 1. A method comprising: receiving, by at least oneprocessing device and from a variable frequency drive electricallycoupled to one or more compressors of a refrigeration system, operationinformation of the one or more compressors; comparing, by the at leastone processing device, the operation information of the one or morecompressors to an operation threshold; determining, by the at least oneprocessing device and based on the comparison, that the operationinformation of the one or more compressors satisfies the operationthreshold; receiving, by the at least one processing device and from oneor more sensors fluidly coupled to the refrigeration system, a value ofat least one of a temperature or a pressure of a working fluid in therefrigeration system; comparing, by the at least one processing device,the value to a fluid parameter threshold that comprises (i) a respectivetemperature threshold of the working fluid that indicates a low loadcondition of the refrigeration system, or (ii) a pressure threshold ofthe working fluid that indicates the low load condition of therefrigeration system; determining, by the at least one processing deviceand based on the comparison, that the value satisfies the fluidparameter threshold by being at or below the fluid parameter threshold;and changing, by the at least one processing device and based on thedetermination that the operation information of the one or morecompressors satisfies the operation threshold or the value of theworking fluid satisfies the fluid parameter threshold, an operationparameter of a component of the refrigeration system, increasing atleast one of: (i) a velocity of the working fluid in a piping assemblyfluidly coupled to the one or more compressors, or (ii) a flow rate ofan oil in the piping assembly flowing into the one or more compressors.2. The method of claim 1, wherein the operation information comprises afrequency of the one or more compressors, and changing the operationparameter comprises changing an operation speed of the one or morecompressors.
 3. The method of claim 2, wherein the frequency comprises afrequency of the one or more compressors over a period of time and theoperation threshold comprises a frequency threshold, the method furthercomprising, before comparing the operation information to the operationthreshold, determining an average frequency of the one or morecompressors over a predetermined period of time, and determining thatthe operation information satisfies the operation threshold comprisesdetermining that the average frequency of the one or more compressors isbelow the frequency threshold.
 4. The method of claim 2, wherein thepiping assembly comprises a suction line fluidly coupled to a fluidinlet of the one or more compressors, the method further comprising:opening, by the processing device and based on the determination thatthe value of the working fluid satisfies the fluid parameter threshold,at least one of (i) a liquid injection valve in fluid communication withthe suction line, or (ii) a gas injection valve in fluid communicationwith the suction line, changing the temperature of the working fluid ina superheated state.
 5. The method of claim 4, wherein the indication ofthe low load condition is in the suction line.
 6. The method of claim 2,wherein the piping assembly comprises a suction line fluidly coupled toa fluid inlet of the one or more compressors, the method furthercomprising, after changing the operation parameter: receiving, by the atleast one processing device and from one or more sensors fluidly coupledto the suction line, fluid information comprising the value of at leastone of the temperature of the pressure of the working fluid in thesuction line; comparing, by the at least one processing device, thevalue of the working fluid to the fluid parameter threshold;determining, by the at least one processing device and based on thecomparison, that the value of the working fluid does not satisfy thefluid parameter threshold; and upon determining that the value of theworking fluid does not satisfy the fluid parameter threshold, repeatingthe steps of claim
 1. 7. The method of claim 1, wherein the pipingassembly comprises a suction line fluidly coupled to the one or morecompressors, and changing the operation parameter comprises changing apressure set point of at least one of a flash tank of the refrigerationsystem or a gas cooler of the refrigeration system, increasing thevelocity of the working fluid flowing in the suction line.
 8. The methodof claim 7, further comprising, after changing the operation parameter:receiving, by the at least one processing device and from one or moresensors of the refrigeration system, a discharge air temperature of anevaporator of the refrigeration system; comparing, by the at least oneprocessing device, the discharge air temperature to a discharge airtemperature threshold; determining, by the at least one processingdevice and based on the comparison, that the discharge air temperaturesatisfies the discharge air temperature threshold; and upon determiningthat the value of the working fluid satisfies the fluid parameterthreshold, repeating the steps of claim
 1. 9. The method of claim 7,further comprising, after changing the operation parameter: receiving,by the at least one processing device and from one or more sensors ofthe refrigeration system, a discharge air temperature of an evaporatorof the refrigeration system; comparing, by the at least one processingdevice, the discharge air temperature to a discharge air temperaturethreshold; determining, by the at least one processing device and basedon the comparison, that the discharge air temperature does not satisfythe discharge air temperature threshold; and resetting, by the at leastone processing device, a timer of the refrigeration system, the at leastone processing device configured to repeat the steps of claim 1 after apredetermined period of time indicated by the timer.
 10. The method ofclaim 7, wherein changing the pressure set point comprises lowering thepressure set point of the flash tank or increasing the pressure setpoint of the gas cooler, increasing the velocity of the working fluid inthe suction line.
 11. The method of claim 1, wherein the piping assemblycomprises a suction line fluidly coupled to the one or more compressors,and changing the operation parameter comprises changing a suction setpoint of the refrigeration system, increasing the velocity of theworking fluid flowing in the suction line.
 12. The method of claim 1,wherein increasing the velocity of the working fluid comprisesincreasing a flow rate of oil return into the one or more compressors.13. The method of claim 1, wherein the piping assembly extends from oneor more evaporators of the refrigeration system to the one or morecompressors, and changing the operation parameter comprises changing theoperation parameter to increase the velocity of the working fluid in thepiping assembly flowing from the one or more evaporators to the one ormore compressors.
 14. The method of claim 13, wherein the pipingassembly comprises a low temperature suction line and a mediumtemperature suction line, the one or more compressors comprises a groupof low temperature compressors and a group of medium temperaturecompressors, and the one or more evaporators comprises a group of lowtemperature evaporators fluidly coupled, through the low temperaturesuction line, to the group of low temperature compressors and a group ofmedium temperature evaporators fluidly coupled, through the mediumtemperature suction line, to the group of medium temperaturecompressors, and changing the operation parameter comprises changing theoperation parameter to increase the velocity of the working fluid in atleast one of the low temperature suction line of the medium temperaturesuction line.
 15. The method of claim 14, wherein the low temperaturesuction line comprises a heat exchanger coil disposed inside a flashtank, the low temperature suction line comprises a uniform diameter, andchanging the operation parameter comprises changing the operationparameter to increase the velocity of the working fluid flowing in thelow temperature suction line from the heat exchanger coil to the groupof low temperature compressors.
 16. The method of claim 1, wherein theoperation information comprises an oil level in an oil separator fluidlycoupled to the one or more compressors, and the operation thresholdcomprises an oil level of the oil separator under the low load conditionof the refrigeration system.
 17. A method comprising: identifying arefrigeration system comprising: one or more compressors, one or moreevaporators, a piping assembly fluidly coupled to and interconnectingthe one or more compressors to the one or more evaporators, a workingfluid configured to flow in the piping assembly from the one or moreevaporators to the one or more compressors; and one or more sensorsfluidly coupled to the refrigeration system, the one or more sensorsconfigured to sense fluid information comprising a parameter of theworking fluid in the refrigeration system; comparing, the parameter ofthe working fluid to a fluid parameter threshold; determining that theparameter of the working fluid satisfies the fluid parameter threshold,wherein the parameter of the working fluid comprises at least one of atemperature of the working fluid, a pressure of the working fluid, or avelocity of the working fluid, and the fluid parameter thresholdcomprises (i) a respective temperature threshold of the working fluidthat indicates a low load condition of the refrigeration system, (ii) apressure threshold of the working fluid that indicates the low loadcondition of the refrigeration system, or (iii) a respective velocitythreshold of the working fluid that indicates a reduced velocity of theworking fluid, and determining that the parameter of the working fluidsatisfies the fluid parameter threshold comprises determining that theparameter of the working fluid is at or below the fluid parameterthreshold; and changing, based on an indication of the temperature ofthe working fluid, an indication of the pressure of the working fluid,or an indication of reduced velocity of the working fluid along thepiping assembly, an operation parameter of a component of therefrigeration system, thereby increasing at least one of: (i) thevelocity of the working fluid in the piping assembly fluidly coupled tothe one or more compressors, or (ii) a flow rate of an oil in the pipingassembly flowing into the one or more compressors.
 18. The method ofclaim 17, wherein the one or more compressors comprises a group ofcompressors and changing the operation parameter of the component of therefrigeration system comprises at least one of: (i) increasing afrequency of a lead compressor of the group of compressors, (ii)lowering a pressure set point of a flash tank of the refrigerationsystem, (iii) increasing a pressure set point of a gas cooler of therefrigeration system, or (iv) lowering a suction pressure or temperatureset point of the refrigeration system.
 19. The method of claim 17,wherein the piping assembly comprises a suction line fluidly coupled toa fluid inlet of the one or more compressors, the method furthercomprising, after changing the operation parameter: opening, based on adetermination that the temperature or the pressure of the working fluidsatisfies the fluid parameter threshold, at least one of (i) a liquidinjection valve in fluid communication with the suction line, or (ii) agas injection valve in fluid communication with the suction line,changing the temperature of the working fluid in a superheated state.20. The method of claim 18, wherein the frequency comprises a frequencyof the one or more compressors over a period of time and an operationthreshold comprises a frequency threshold, the method further comprises:determining an average frequency of the one or more compressors over apredetermined period of time; and determining that the operationparameter satisfies the operation threshold comprises determining thatthe average frequency of the one or more compressors is below thefrequency threshold.